Packet telephone networks have the potential to replace current circuit-switched telephone networks. For example, Hybrid Fiber Coax (HFC) networks have the potential to replace the current telephone local loop. To achieve this goal, the HFC telephone network needs to provide services similar to services on the current circuit-switched telephone network.
One such service is the ability of the current telephone network to carry FAX and voice band modem data. To be competitive with current telephone networks, the HFC telephone network should be capable of carrying FAX and voice band modem data at a performance level comparable to that of the current circuit-switched telephone network. The ability of the HFC network to carry FAX and voice band modem data depends on its ability to minimize frame slips.
Another service on current telephone networks allows the current telephone network to deliver high quality speech free of interference (e.g., pops and clicks). To be competitive with current telephone networks, the HFC telephone networks should be capable of delivering high quality speech at a performance level comparable to that of the current circuit-switched telephone network. The ability of the HFC network to deliver high quality speech depends on its ability to minimize packet loss. This is particularly true when speech compression is used to reduce bandwidth.
Networks may often be classified as synchronous, plesiochronous, or asynchronous. In synchronous networks the sampling, reconstruction and the transport mechanism operate at exactly the same rate. However, there may be a phase difference between clocks within the network due to propagation time delays or jitter introduced by the network. In a synchronous network, clocks are traceable to one primary reference clock (PRC). The accuracy of the PRC is typically better than +/−1 in 1011 and is derived from a cesium atomic clock (i.e. a Stratum 1 level).
In plesiochronous networks, the sampling, reconstruction and the transport mechanisms operate at almost the same rate with any variations being constrained within a tight tolerance. Also, the two networks derive their clocks from two different PRCs. Although these clocks are extremely accurate, there is a difference between clock periods. Typical clock accuracy of plesiochronous networks is between +/−1.6 in 108 (i.e. a Stratum 2 level) and +/−4.6 in 106 (i.e. a Stratum 3 level).
In the case of asynchronous networks, the sampling, reconstruction and the transport mechanisms operate at nominal rates. In asynchronous networks, the difference between two clocks is much greater than the plesiochronous clock difference. For example, if two clocks are derived from free-running quartz oscillators, they can be described as asynchronous. Asynchronous networks typically maintain clock accuracy less than the Stratum 3 level.
Depending upon the network or networks used, frame slip or packet loss may occur. A frame refers to the amount of data associated with a speech sample, for example, a G.711 CODEC speech sample. When a frame slip occurs, speech is lost.
A packet refers to the amount of data carried between a header and trailer. For example, a VoIP packet typically contains many speech samples and a packet loss results in the loss of many speech samples. In another example, a G.711 CODEC samples speech every 125 microseconds, aggregating eighty samples into a single packet generated every 10 milliseconds. Therefore, a packet loss results in the loss of eighty G.711 CODEC speech samples and this represents eighty frame slips.